chore: prevent visitors from breaking demo (#2869)

* chore: add en-GB language

* chore: remove en-GB contents

* chore: prevent visitors from breaking demo
- prevent disabling password login
- prevent updating `memos-demo` user
- prevent setting additional style
- prevent setting additional script
- add some error feedback to system settings UI

* Revert "chore: add en-GB language"

This reverts commit 2716377b04.

api/v1/user.go

@@ -316,6 +316,14 @@ func (s *APIV1Service) DeleteUser(c echo.Context) error {
 		return echo.NewHTTPError(http.StatusBadRequest, "Cannot delete current user")
 	}
 
+	findUser, err := s.Store.GetUser(ctx, &store.FindUser{ID: &userID})
+	if err != nil {
+		return echo.NewHTTPError(http.StatusInternalServerError, "Failed to find user").SetInternal(err)
+	}
+	if s.Profile.Mode == "demo" && findUser.Username == "memos-demo" {
+		return echo.NewHTTPError(http.StatusForbidden, "Unauthorized to delete this user in demo mode")
+	}
+
 	if err := s.Store.DeleteUser(ctx, &store.DeleteUser{
 		ID: userID,
 	}); err != nil {
@@ -366,6 +374,10 @@ func (s *APIV1Service) UpdateUser(c echo.Context) error {
 		return echo.NewHTTPError(http.StatusBadRequest, "Invalid update user request").SetInternal(err)
 	}
 
+	if s.Profile.Mode == "demo" && *request.Username == "memos-demo" {
+		return echo.NewHTTPError(http.StatusForbidden, "Unauthorized to update user in demo mode")
+	}
+
 	currentTs := time.Now().Unix()
 	userUpdate := &store.UpdateUser{
 		ID:        userID,